Continuous Furnace

(or holding furnace), a straight-away furnace for heating metal billets before rolling, forging, or stamping. In such furnaces the billets are arranged crosswise and move in the opposite direction to the products of combustion of the fuel. This type of motion provides high efficiency of use of the heat supplied to the furnace. The billets pass consecutively through three heat-treatment zones: preheating, heating, and soaking (temperature equilibration). The heating zone may consist of a few consecutive sections, with introduction of additional fuel into each section. The soaking zone is not necessary for billets of small cross section.

Continuous furnaces are classified according to the number of heating zones (two to five), the method of moving the heated billets (pusher-type and walking-beam furnaces), and special structural features (underside-heating and uphill furnaces). The furnaces are heated by gaseous or liquid fuel using burners or atomizers located mainly on the back walls of the heating and soaking zones; less frequently the burners are attached to the side walls and furnace roof.

The temperature in continuous furnaces is constant over time and variable over the length of the furnace. In the heating and soaking zones the temperature is almost constant; in the holding zone it decreases toward the front of the furnace.

Pusher-type continuous furnaces for heating bars up to 120 mm thick are made with a slanted hearth (uphill furnaces); furnaces for heating larger bars are made with a horizontal hearth. Underside heating, which provides heating from two sides, is used for bars more than 120 mm thick.

Walking-beam continuous furnaces are promising. In such furnaces there is a gap between the billets, and they are heated on three or four sides, thus, the heating is more rapid and uniform and oxidation and decarburization of the metal are reduced. A walking-beam continuous furnace is a highly mechanized industrial complex. Such furnaces may be easily cleared of billets during repairs and shutdowns. Labor-intensive manual cleaning of the hearth is not necessary in the operation of these furnaces.

Continuous furnaces usually have waste-heat boilers, as well as recuperators for heating air or a mixture of air and gas.

The main characteristics of continuous furnaces are shown in Table 1.

an industrial furnace with continuous operation designed to heat individual items; it features a mechanized transport to move the items through the furnace. Continuous furnaces are classified according to their mode of transporting items. Thus, in pusher and tunnel furnaces, items are pushed along; in furnaces with a roller hearth and fast-heating sectional furnaces, items are moved on a roller conveyor; in walking hearth furnaces, items are transported by mobile beams; in car-bottom furnaces, items are moved on a conveyor; and in rotary-ring furnaces, items are moved on a revolving hearth.

Continuous furnaces are used for heating metal billets prior to pressure treatment, for heat-treating items and parts, and for firing ceramic- and enamel-coated metal objects. The working chamber of most continuous furnaces is relatively small in cross section (1–6 m in width, sometimes up to 25 m, with a height of 1-2 m) but quite long (up to 250 m). The heating profile is constant over time but varies over the length of the furnace. The temperature is uniform in a given cross section of the furnace but may vary over the length of the furnace.

Continuous furnaces are heated by many relatively small heat sources located mainly on the sidewalls; in some cases, the heat sources are located on the roof and in the hearth. When gas is used to heat items above 700°C or mazut is used to heat items above 1000°C, the fuel is burned directly in the working chamber. When gases having a low heat of combustion are used for high-temperature heating, the gas or air is preheated. Low-temperature continuous furnaces are heated by burning gas or mazut in fireboxes located in the furnace sidewalls, underneath the hearth, or above an interjacent lattice roof; the products of complete-combustion are then directed into the working chamber. In order to obtain high uniformity of heating, the combustion products are recirculated. Continuous furnaces with a controlled atmosphere or furnaces in which the items treated are not permitted to come in contact with the combustion products of the fuel are heated by radiant tubes or by electrical resistance heaters.

The specific duration of heating in continuous furnaces (except for sectional types) at a heating temperature of 700°–1150°C varies from 10 to 15 min for each centimeter of thickness of the item treated; in sectional furnaces, this value is 1–3 min/cm. The heating capacity of continuous furnaces per square meter of hearth at a heating temperature up to 300°C is approximately 200 kilowatts; for higher temperatures, the value may reach 300 kilowatts. A promising type of continuous furnace that transports items on a gas cushion features jet heating and cooling.